Measuring the level of a liquid, fluids, granular compounds or other materials stored in a tank is a basic industrial need. Several technologies have historically been employed and the last decade radar has emerged as the dominating method for high quality measurements. These devices utilize antennas to transmit electromagnetic waves toward the material being monitored and to receive electromagnetic echoes which are reflected at the surface of the material being monitored. Today, numerous units for radar level gauging are installed worldwide, and among liquid level gauges the radar method has a growing percentage. The radar level gauges (RLGs) are typically mounted in tanks which can have a height from a few meter to 20–30 m or even more and with volumes from a few m3 and up to 100 000 m3 or more, typically located outdoors. High accuracy and reliability has been important virtues for RLGs.
Frequencies around 6, 10 and 26 GHz have typically been used, but other frequencies, such as in higher frequency bands, are feasible as well, such as near 60 and 80 GHz. Some RLGs use FMCW as the radar method (with a 1–2 GHz sweep, other use very short pulses (e.g. 0.5–1 ns) and it is expected that other existing radar distance measuring methods may be employed as well.
A radar level gauge radiates microwave power and to get desired accuracy the occupied bandwidth is conventionally very big as compared to conventional radar such as surveillance radar on ships etc. Generally, due to the wide bandwidth practically no radiation in free space is allowable, since the outdoor frequency space is already sliced in narrow bands and designated for various types of use. There are no multi-GHz bands free and unused, at least not below 100 GHz.
RLGs are in use often fixedly mounted in structures such as, in the typical case, a closed metallic tank. Typically there is a need to keep the tank well closed, due to pressure differences, risk for leakage, etc. However, of the small microwave power used inside the tank a small fraction of it may escape from the tank and seen from the outside as emitted electromagnetic radiation without apparent directivity. The electromagnetic radiation outside the tank caused by the RLGs could thus appear like spurious radiation. Approvals for existing RLGs have among other been based on emission outside the tank which is below the same level as any kind of electronics is allowed to emit according to current EMC-rules. In a specific country or region this could mean emissions below 0.075–1 microwatt outside of the tank which by experience is feasible in conventional designs.
However, special measuring needs could require use of higher power etc. in RLGs, e.g. to measure longer distances, which could give an increase of microwave leakage when using the conventional designs of today. There could also be situations in which one would like to reduce the amount of escaping microwave power; e.g. local occurrence of special conditions or especially sensitive equipment are potential reasons to further reduce the amount of escaping MW power.
Thus, even if the RLG systems in use today generally have passed applicable type approval based on low emission outside the tank there is foreseen situations where a reduction of the emission is desirable, and situations could occur where one would like to reduce the amount of escaping microwave power.